Field of the Invention
The present invention relates to a denitrification apparatus, and more particularly, to a denitrification apparatus that reduces nitrogen oxides included in exhaust gas discharged from combustion facilities.
Description of the Related Art
Nitrogen oxides (NOx) in exhaust gas discharged from thermal power plants and various plants serve as a causative agent of photochemical smog and acid rain, therefore, a fuel gas denitrification method by selective catalytic reduction using ammonia (NH3) as a reducing agent is widely used as a removal technique of the nitrogen oxides. The selective catalytic reduction is performed by injecting NH3 to NOx included in exhaust gas as shown in an expression (1) below to reduce NOx by using a denitrification catalyst.4NO+4NH3+O2→4N2+6H2O   (1)
In recent years, in the United Stated, a case of transporting ammonia in the form of ammonia water becomes usual in order to safely transport ammonia serving as a reducing agent. In a case where ammonia water is used as a reducing agent, usually the ammonia water is sprayed into an exhaust gas duct after temporarily vaporized by using a heat source such as a heater to allow a reaction according to the expression (1) to occur. In that case, however, considerable energy is required to vaporize the ammonia water by using a heat source. On the other hand, in recent years, apparatuses have begun to be installed which are provided with a system of vaporizing ammonia water by atomizing the ammonia water to be directly sprayed into an exhaust gas duct so that the ammonia water is directly vaporized because of the high temperature heat of exhaust gas (refer to Patent Literature 1: International Publication No. WO 01/012299, for example). In the system, since it is required to spray ammonia water into an exhaust gas duct by atomizing the ammonia water, an injection nozzle including a two- fluid nozzle for spraying the ammonia water and compressed air for spraying the ammonia water is generally used.
In addition, in equipment in a thermal power plant, exhaust gas from a coal-fired boiler contains many ashes, so that deposits of the ashes are produced in a bottom of an exhaust gas duct installed in a horizontal portion.
In a case where ammonia water is sprayed into an exhaust gas duct after temporarily vaporized by using a heat source such as a heater, considerable energy is required to vaporize the ammonia water by using the heat source, resulting in low energy efficiency.
In addition, like the invention described in Patent Literature 1, in a case where ammonia water is atomized to be directly sprayed into an exhaust gas duct so that the ammonia water is directly vaporized because of the high temperature heat of exhaust gas, a heat source for vaporizing the ammonia water and a dilution fan for diluting vaporized ammonia become unnecessary. As a result, initial apparatus cost for such a facility and operating cost are reduced. However, since concentration of NOx contained in exhaust gas is reduced due to low NOx combustion and the like in recent years, an amount of ammonia water required by the denitrification apparatus is reduced. As above, if a supply amount of ammonia water to be supplied into an exhaust gas duct is reduced, ammonia water stayed in a lance and an injection nozzle, which spray the ammonia water into the exhaust gas duct, tends to evaporate. Bubbles produced by the evaporation of the ammonia water cause unstable amounts of ammonia water sprayed from the injection nozzle to obstruct steady atomizing and spraying, thereby reducing denitrification performance.
As shown in FIG. 16, for example, there is a case where concentration of NOx contained in exhaust gas in an exhaust gas duct 101 is low due to low NOx combustion and the like, or a case where ammonia water is required to be injected into an exhaust gas duct 101 even at the time of operating at a low load, due to tightening of NOx emission regulation. In the cases above, the ammonia water is heated by temperature of exhaust gas and the like before the ammonia water reaches a spray nozzle 102 with reduction in ammonia water used in denitrification reaction, which causes a part of the ammonia water to evaporate, whereby bubbles occur. As a result, an amount of the ammonia supplied to the spray nozzle 102 becomes intermittent and unstable, thereby reducing denitrification performance.
An object of the present invention, in light of the above-mentioned problem, is to provide a denitrification apparatus capable of supplying an amount of a reducing liquid corresponding to concentration of nitrogen oxides in exhaust gas to a nozzle provided in an exhaust gas duct regardless of a load of a combustion facility, and preventing denitrification performance thereof from deteriorating by preventing vaporization of the reducing liquid in a reducing liquid supply unit for supplying the reducing liquid to the nozzle.